Wireless communication systems, including cellular phones, paging devices, personal communication services (PCS) systems, and wireless data networks, have become ubiquitous in society. Wireless service providers continually try to create new markets for wireless devices and to expand existing markets by making wireless devices and services cheaper and more reliable. The price of end-user wireless devices, such as cell phones, pagers, PCS systems, and wireless modems, has been driven down to the point where these devices are affordable to nearly everyone and the price of a wireless device is only a small part of the end-user's total cost. To continue to attract new customers, wireless service providers concentrate on reducing infrastructure costs and operating costs, and on increasing handset battery lifetime, while improving quality of service in order to make wireless services cheaper and better.
To maximize usage of the available bandwidth, a number of multiple access technologies have been implemented to allow more than one subscriber to communicate simultaneously with each base station (BS) in a wireless system. These multiple access technologies include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA) These technologies assign each system subscriber to a specific traffic channel that transmits and receives subscriber voice/data signals via a selected time slot, a selected frequency, a selected unique code, or a combination of these factors.
CDMA technology is used in wireless computer networks, paging (or wireless messaging) systems, and cellular telephony. In a CDMA system, mobile stations and other access terminals (e.g., pagers, cell phones, laptop PCs with wireless modems) and base stations transmit and receive data on the same frequency in assigned channels that correspond to specific unique orthogonal codes. For example, a mobile station may receive multiple forward channel data signals from a base station. Each forward channel signal transmitted by the base station is formatted, repeated, encoded, formatted, interleaved, spread with a Walsh code assigned to that channel, and separated into an in-phase (I) digital stream and a quadrature (Q) digital stream. The I-digital stream is further spread by an in-phase pseudo-noise (PN) code. The Q-digital stream is spread by a quadrature pseudo-noise code. The PN-modulated I-digital stream and the PN modulated Q-digital stream are filtered by a finite impulse response (FIR) filter and then used for subsequent quadrature phase-shift keying (QPSK) modulation of a radio frequency (RF) carrier signal.
In another embodiment, a base station may receive reverse channel data signals from mobile stations that are formatted, repeated, encoded, block-interleaved, and spread prior to transmission by the mobile station with a pseudo-noise code based on the mobile station identification number. In another embidiment, the data signals may be encoded with 64-ary or M-ary modulation prior to block interleaving. Those skilled in the art will recognize that a mobile station may employ quadrature phase shift keying (QPSK) modulation, binary phase shift keying (BPSK) modulation, quadrature amplitude modulation (QAM) or other digital modulation format for modulation of a radio frequency (RF) carrier for transmission of the data signals. One such implementation, is found in the TIA/EIA-95 CDMA standard (also known as IS-95). Another implementation is the TIA/EIA-2000 standard (also known as IS-2000).
Power amplifiers for amplification of CDMA signals have traditionally been some of the most expensive components of a wireless communication system and have often resisted attempts aimed at lowering their cost. A key design specification for power amplifiers used for amplification of CDMA signals used in cellular radio systems is limitation on adjacent channel power (ACP). ACP is a result of signal distortion caused by the non-linearity of the devices used in power amplifiers that produces undesired spectral components in adjacent transmission channels. Usually, devices are more linear with lower input signal levels. That is, for a given device, signals with less strength experience less distortion and, thus, have lower ACP. A transmitter (or power amplifier) may need to operate in back-off mode (reduced input signal level) to reduce signal distortion in order to meet the ACP requirements. However, the required amplifier back-off to accommodate large peak power signals leads to higher cost, inefficient operation and excessive heat dissipation—all of which combine to raise system cost significantly. It has therefore been the goal of designers to reduce peak-to-average power ratios as much as possible without degrading other performance parameters.
For example, U.S. Pat. No. 5,930,299 discloses a digital modulator with compensation that reduces the peak-to-average power ratio of a modulated signal to allow an increase in power amplifier efficiency. In the digital modulator with compensation, a digital bit stream is sent to an encoder which translates bit sequences into I and Q digital pulses. These target symbol sequences have been predetermined to cause excessive amplitude peaking in the modulated signal. If a target symbol sequence is encountered, it is adjusted using amplitude or filter compensation. The compensation is implemented by adjusting the coefficients of I and Q finite impulse response (FIR) pulse-shaping filters for one or more symbols in each target symbol sequence. Non-target symbol sequences in the digital bit stream are largely unaffected. The I and Q FIR pulse-shaping filters are followed by I and Q digital-to-analog converter (DAC) and reconstruction filters. Together, the filters shape the I and Q pulses according to communication system specifications. The filtered I and Q signals are then sent to a quadrature modulator for RF modulation, amplified, and transmitted over a communication channel. Howver, the device disclosed in U.S. Pat. No. 5,930,299 requires repeated adjustment of the I and Q FIR pulse-shaping filter coefficients, which is processor intensive and which may not yield adequate filtering of ACP for all combinations of Walsh or spreading codes.
U.S. Pat. No. 5,838,732 discloses a wideband digital combiner that generates a composite signal as a frequency multiplexed combination of many narrowband modulated digital carrier signals. The technique used by the digital combiner involves introducing predetermined phase shifts into each of the digital channel signals after a baseband modulation step. The wideband composite signal thus exhibits a reduced peak-to-average signal power, despite the fact that the phases of the digital carrier signals cannot be directly controlled. This permits the use of a power amplifier, which may have a much smaller peak-to-average rating. However, those skilled in the art will recognize that the phase shifting of the digital channel signals degrades the detection and demodulation of the QPSK signals at the receiver by introducing phase jitter or zero-crossing jitter. Furthermore, it decreases the auto-correlation magnitude of desired signals and increasing the cross-correlation magnitude of undesired signals thereby increasing the level of interference.
U.S. Pat. No. 5,606,578 discloses a communication device in which a digital processor responds to an information generator by alternately mapping the digital information onto a first or a second constellation diagram to produce data symbols. The processor processes the data symbols alternately from the first and second constellation diagrams in order to minimize the peak-to-average power ratio at the amplifier. This alternating scheme allows the power amplifier to operate more efficiently. However, this mapping to different constellations to produce data symbols changes the Walsh modulation sequence at the receiving station and thereby removes the unique identification for the traffic channel.
Therefore, there is a need in the art for improved CDMA-based wireless devices that minimize peak-to-average power ratio without suffering the other performance degradations associated with the prior art systems.